how bake three different materials belonging to an object into one single textured one
After some correction I got the baked image.



Second Figure is original .blend. There were some errors.
The Map name on the right is painted with a caution color. In addition to it, there are blank names in Map of other textures. 
Blender is not specified what to use from multiple UV maps. 
And perhaps the active material of object _2B.004 is UVMat.2A.